In a mobile communication system according to the CDMA (Code Division Multiple Access) method, a radio base station simultaneously receives user signals from a plurality of mobile stations, and signals of other users therefore interfere with the signal of a particular user. An adaptive antenna is used to receive a desired user signal at a higher gain.
An adaptive antenna is composed of a plurality of antennas and controls amplitude and phase in accordance with a complex number weight that is conferred to the signals that are received by each of the antennas to form directivity. An adaptive antenna is thus capable of suppressing other user signals that constitute interference and of effectively receiving a desired user signal.
Two methods are typically used for determining the weight that is conferred to each antenna of an adaptive antenna.
In one method, weighting is determined by performing feedback control using an algorithm that follows the MMSE (Minimum Mean Square Error) standard. An adaptive updating algorithm such as an RLS (Recursive Least Square) algorithm of the sequential weight updating type or a representative LMS (Least Mean Square) algorithm is used.
In contrast, the other method is the open-loop control method that is the object of the present invention. According to this method, an arrival direction estimation algorithm such as a MUSIC (MUltiple Signal Classification) algorithm or an ESPRIT (Estimation of Signal Parameters via Rotational Invariance Techniques) algorithm is used to estimate the arrival direction of a desired wave based on the signals received in antennas, and the weight of each antenna is then determined in accordance with this direction. A method of determining weight by open-loop control is disclosed in, for example, JP 11-274976-A.
FIG. 1 is a block diagram showing an example of the configuration of an adaptive antenna reception device of the prior art that is described in JP 11-274976-A. In the adaptive antenna reception device that is shown in FIG. 1, N is the number of antennas that make up the adaptive antenna (where N is an integer equal to or greater than 2), and L is the number of synthesized multi-paths (where L is a natural number). FIG. 1 shows the circuit portion for receiving the user signal that is received from the mobile station of the kth user (where k is a natural number).
Referring to FIG. 1, the adaptive antenna reception device includes: antennas 11-1N, signal processors 21-2L, adder 11, determiner 12, and searcher 16. Fingers that correspond to each of the multi-paths that undergo rake synthesis are assigned to signal processors 21-2L.
Signal processor 21 includes: delay unit 31, despreading circuits 411-41N, weighting/synthesizing circuit 51, weight calculation unit 61, transmission path estimation circuit 71, complex conjugate circuit 81, initial weight generation unit 91, and multiplier 101. In addition, weight calculation unit 61 includes: signal common-mode average calculation unit 131, correlation detection unit 141, and time average calculation unit 151.
Although not shown in the figure, the interiors of signal processors 22-2L have the same configuration as signal processor 21. For example, signal processor 22 is made up from: delay unit 32, despreading circuits 421-42N, weighting/synthesizing circuit 52, weight calculation unit 62, transmission path estimation circuit 72, complex conjugate circuit 82, initial weight generation unit 92, and multiplier 102. In addition, weight calculation unit 62 includes signal common-mode average calculation unit 132, correlation detection unit 142, and time average calculation unit 152.
Searcher 16 uses each of the signals that are received by N antennas 11-1N to detect the delay time of L multi-paths. Searcher 16 then reports the timing information of the delay times that are used in each finger to delay units 31-3L, weight calculation units 61-6L, and initial weight generation units 91-9L of signal processors 21-2L that constitute each finger of rake synthesis. The N antennas 11-1N are arranged in proximity so as to have a high level of correlation with each other, and as a result, the delay profiles of N antennas 11-1N can be considered to be identical. Accordingly, the timing information of the delay time of each multi-path can be used in common for all of antennas 11-1N.
Delay unit 31 delays each of the signals that are received by antennas 11-1N in accordance with the timing information that has been reported from searcher 16 and sends the signals to despreading circuits 411-41N. Delay units 32-3L similarly delay each of the signals that have been received by antennas 11-1N in accordance with the timing information that has been reported from searcher 16. In this way, signal processors 21-2L can each be placed in correspondence with the L multi-paths.
Despreading circuits 411-41N perform despreading of each of the received signals that have been delayed by delay unit 31 and send the results to weighting/synthesizing circuit 51, weight calculation unit 61, and initial weight generation unit 91.
Initial weight generation unit 91 generates an initial weight for use when weight of sufficient accuracy cannot be obtained by weight calculation unit 61 and sends the result to weighting/synthesizing circuit 51.
Initial weight generation unit 91 is used when searcher 16 newly assigns a finger to signal processor 21, or when sufficient averaging time cannot be secured in weight calculation unit 61 of signal processor 21 to which a finger has been assigned. Averaging time is the time that is used for finding the average for the variation value that is the object of averaging. The average value in the averaging time of the variation value is found by averaging in the averaging time. In addition, initial weight generation unit 91 is also used when the path timing of a finger that is in use undergoes a large change.
FIG. 2 is a block diagram showing the configuration of weighting/synthesizing circuit 51. Weighting/synthesizing circuit 51 includes: multipliers 171-17N, adder 18, and complex conjugate circuits 191-19N.
Complex conjugate circuits 191-19N of weighting/synthesizing circuit 51 generate a complex conjugate of the weight that is generated by weight calculation unit 61 or by initial weight generation unit 91 and send this complex conjugate to multipliers 171-17N.
Each of multipliers 171-17N multiplies received signals that have undergone despreading by despreading circuits 411-41N with the complex conjugates of weights that have been generated by complex conjugate circuits 191-19N that correspond to the signals and sends the results to adder 18.
Adder 18 synthesizes the outputs of multipliers 171-17N and sends the results to transmission path estimation circuit 71 and multiplier 101 that are shown in FIG. 1.
Signal common-mode average calculation unit 131 of weight calculation unit 61 adds vectors of the symbols of each signal that has undergone despreading by despreading circuits 411-41N during the matching phase, finds the average values of signals for each antenna, and sends the results to correlation detection unit 141. At this time, any number of symbols may undergo vector addition, and any weight can be applied to each symbol.
Correlation detection unit 141 uses the average value of each signal from signal common-mode average calculation unit 131 to find correlation between the received signal at the antenna that is the standard and received signals at other antennas. For this purpose, correlation detection unit 141 multiples the complex conjugate of the average value of the signal that corresponds to the reference antenna by the average values of signals for other antennas and sends the correlations that are the results of each multiplication to time average calculation unit 151.
Time average calculation unit 151 takes the mean in a prescribed time interval for each multiplication result from correlation detection unit 141, finds the weight for each of antennas 11-1N, and sends the results to weighting/synthesizing circuit 51. There are a variety of weight methods and time intervals for taking the mean in time average calculation unit 151, and the method and time interval can be freely selected.
In this way, weighting/synthesizing circuit 51 uses the weight that has been generated in weight calculation unit 61 to control and synthesize the amplitude and phase of the signals received by antennas 11-1N and form the directivity by which a desired user signal can be received at high gain.
Transmission path estimation circuit 71 estimates the transmission path distortion based on the output signal of weighting/synthesizing circuit 51 and sends the result to complex conjugate circuit 81.
Complex conjugate circuit 81 generates a complex conjugate of the transmission path distortion that has been estimated by transmission path estimation circuit 71.
Multiplier 101 multiplies the complex conjugate of the transmission path distortion that has been generated by complex conjugate circuit 81 by the output signal of weighting/synthesizing circuit 51 to compensate the transmission path distortion.
Signals in which the transmission path distortion from each finger has been compensated are similarly obtained by signal processors 21-2L.
Adder 11 performs rake synthesis by adding the output signals of signal processors 21-2L and sends the synthesized output signal to determiner 12.
Determiner 12 determines each symbol and supplies the reception symbol of kth user as output.
FIG. 3 is a flow chart showing the operation when assigning fingers in the adaptive antenna reception device that is shown in FIG. 1. Referring to FIG. 3, signal processors 21-2L first determine whether an assigned finger is a new finger or not (Step C1).
If the assigned finger is a new finger, signal processors 21-2L use the initial weight that has been generated by initial weight generation units 91-9L in weighting/synthesizing circuits 51-5L (Step C4).
If the assigned finger is not a new finger, signal processors 21-2L determine whether sufficient averaging time has been secured in signal common-mode average calculation units 131-13L and time average calculation units 151-15L of weight calculation units 61-6L (Step C2).
If sufficient averaging time of weight calculation units 61-6L has not been secured, signal processors 21-2L use the initial weight that has been generated in initial weight generation units 91-9L in weighting/synthesizing circuits 51-5L (Step C4). On the other hand, if sufficient averaging time of weight calculation units 61-6L has been secured, signal processors 21-2L use the weight that was generated in time average calculation units 151-15L in weighting/synthesizing circuits 51-5L (Step C3).
FIG. 4 is a flow chart showing the operations at the time of change of path timing of a finger in the adaptive antenna reception device that is shown in FIG. 1. Referring to FIG. 4, signal processors 21-2L first determine whether the path timing of a finger has changed by xT chips or more (Step D1). The value xT is the threshold value for the amount of change in path timing, and determines whether the rate of change in path timing is at a level that cannot be followed by the weights that are calculated by weight calculation units 61-6L.
When the path timing of a finger equals or exceeds xT chips, signal processors 21-2L use the initial weights that are generated by initial weight generation units 91-9L in weighting/synthesizing circuits 51-5L (Step D4). When the change in path timing of a finger falls short of xT chips, signal processors 21-2L determine whether sufficient averaging time has been secured in signal common-mode average calculation units 131-13L and in time average calculation units 151-15L of weight calculation units 61-6L (Step D2).
If the averaging time of weight calculation units 61-6L is not sufficient, signal processors 21-2L use the initial weights of the finger that have been generated by initial weight generation units 91-9L in weighting/synthesizing circuits 51-5L (Step D4). On the other hand, if sufficient averaging time of weight calculation units 61-6L has been secured, signal processors 21-2L use the weights of the finger that have been generated in time average calculation units 151-15L of weight calculation units 61-6L in weighting/synthesizing circuits 51-5L (Step D3).
Generally, two methods are used in adaptive antennas for determining the initial weight.
One method takes into consideration the point that the arrival direction of a user signal differs due to reception conditions, and takes as the initial weight a value such as a non-directional weight that allows reception regardless of the reception conditions in order to enable reception of the user signal under any conditions.
The other method is a method of estimating the initial weight based on signals that are received by a plurality of antennas 11-1N (refer to JP 2002-77011-A). In this method, the transmission path is estimated based on, for example, the received signals from each antenna, and the thus-obtained weight is taken as the initial weight.
The above-described methods of the prior art have the following problems:
When a non-directional weight is taken as the initial weight, gain is the same for all directions, and as a result, a beam cannot be directed in the arrival direction of a desired user signal.
Using signals that are received by a plurality of antennas 11-1N to estimate the transmission path and then using the thus-obtained weight as the initial weight complicates highly accurate estimation of the transmission path in a short time interval, and therefore prevents direction of a beam in the arrival direction of a desired user signal.
As a result, when the path timing of a finger changes greatly at the time a finger is newly assigned, or when sufficient averaging time cannot be secured in weight calculation units 61-6L, there is a potential for degradation of the reception characteristics of the user signal.
Using signals that are obtained by a plurality of antennas 11-1N to estimate the transmission path and then using the thus-obtained weight as the initial weight, not only entails a large amount of calculation for estimating the transmission path, but also increases the burden placed on signal processors 21-2L and demands a high level of processing capability.